Summary Direct germline editing in donor spermatogonial stem cells will streamline production of genetically modified model organisms and provide unprecedented quality control to accurately predict germline transmission rates for individually selected, or multiple, co-selected targeted alleles. GenomeDesigns' direct germline editing technologies will transform the custom rat model market by providing the broadest range of targeted genomic DNA insertion sizes available in the broadest range of tractable rat genetic backgrounds. Here, we propose three specific aims to produce frozen spermatogonial stocks that can be used as germline vectors to build custom designed, genetically modified rat models. In Specific Aim 1, spermatogonial stocks derived from up to 12 different popular outbred and inbred rat genetic backgrounds will be generated to seed GenomeDesigns with the most comprehensive catalogue of custom designable rat models. In Specific Aim 2, precise spermatogonial gene targeting efficiencies will be measured by droplet digital PCR (ddPCR) to guide animal production protocols, benefiting individual customers and GenomeDesigns. We will apply ddPCR to project real production cost and time savings received by co-transmitting, co-selected gene targeting events from an individual germline, or by polyclonal rat production where distinct spermatogonial lines are pooled and transplanted into a single recipient rat. In Specific Aim 3, developing recipient males compatible with diverse donor spermatogonial strains will build an unmatched portfolio of biomedically relevant rat strains amenable to genetic engineering. By completing Phase I Specific Aims, GenomeDesigns will provide proof-of-concept for building a genetically diverse catalogue of spermatogonial stocks that uniquely enable strong genetic selection for precise, sexually transmittable, genomic modifications directly in the rat germline. Benchmarks for this Phase I SBIR project are: 1) Significantly expand spermatogonial gene targeting services into new rat genetic backgrounds 2) Establish ddPCR as prognostic for germline transmission of co-selected gene targeting events 3) Produce recipient rat models for diverse donor rat spermatogonial genetic backgrounds